The original Xbox was such a special system. Its controller, its size, its UI and its library were all so unique in terms of style and vibe. Truly felt like a next-gen system to lil ole me. Halo CE was the first FPS that clicked with me enough to make me a lifelong fan of the genre and as a Sega kid I got tons of goodies that kept me glued to the system. Felt like nearly every game came with a demo section full of trailers and playable snippets that helped clue me in on games to actually seek out and ask my parents for since I didn't have magazines growing up. With other systems I had no idea what was coming out until I'd see it on a store shelf, but with Xbox I was actively looking forward to new releases for the first time ever! Halo 2 was the first game I ever pre-ordered and I still remember how exciting it was walking into EB Games to snag my copy.
Microsoft being willing to throw a stack at just about anyone who promised them exclusivity led to a lot of junk but it also led to plenty of gold. It was also the first system to really get me into big western RPGs with games like Jade Empire, Morrowind and Knights of the Old Republic. Was also the first time I got to really experience just how different the same game could be on multiple systems, since I'd play games like Sonic Heroes or Star Wars Battlefront 2 on my Xbox only to be let down whenever I'd go to a friend's house and have to deal with such blatantly inferior versions.
Phantom Dust also remains one of the most unique and stellar games of all time. Forever sad that Microsoft opened their Japan game studio, let them make 2 amazing games (one of which they never localized) and then shuttered them before the 360 even launched. Sadder still that the remake got canned :(
God, I also remember spending hours slowly burning tons of CDs to the system to use it as a makeshift CD player, only to discover that some games let me play the tunes in-game! Blew my mind as a kid and made Burnout 3 go from excellent to un-fucking-touchable.
I'm also just a fan of the Duke to be honest. I totally respect that both the Duke and the S revision controllers are unwieldy and that things like that black/white buttons were half-hearted attempts at a 6-button layout that didn't work, but to this day I prefer playing my OG Xbox games and even modern Halo on a Duke.